How do i change the size of the box for my collision bounds?
Try changing the drop down menu labeled Box under the collision bounds panel to Convex Hull, or Triangle Mesh if the first doesn't work. Also, if it is still falling through the floor, try upping the substeps from 1 to 5 under Properties Editor>World>Physics>Physics Steps:>Substeps. If none of this helps, or if it does and you want to learn more, Blender Cookie has some good game engine tips at https://www.youtube.com/watch?v=QY7iJmqsJf8

Hope this helps and happy Blending!